(1) Field of the Invention
The present invention relates to a display panel used for displaying images, such as for a computer or television, especially a display panel with silver electrodes formed on the panel surface, and to a production method for such a display panel.
(2) Description of Prior Art
In the field of image displays for computers, televisions and other devices, recently field emission display panels, plasma display panels (PDP) and other types of display panel have received increasing attention as devices that allow a large color image display in a thin package. The PDP especially, because of its excellent high speed response and wide viewing-angle characteristics, has become the object of great activity, as companies and research institutions step up RandD efforts aimed at a mass market.
A PDP has a front glass substrate and a back glass substrate, separated by barrier ribs. A plurality of display electrodes are formed in a stripe pattern on the back of the front glass substrate (the side facing the back glass substrate), and a dielectric layer is formed covering the electrodes.
In a conventional PDP, the front glass substrate is made from a soda-lime-borosilicate glass sheet, and the display electrodes are Crxe2x80x94Cuxe2x80x94Cr or silver, which are relatively easily formed.
A silver electrode can be formed by thin-film method, but the relatively low-cost thick-film method is used also. The first step in the thick-film method is to form a thick silver film in the shape of the electrode pattern, by applying a silver paste containing silver particles, glass flit, resin, solvent and such to the front glass substrate by a screen printing process, or by affixing a film containing silver particles, glass flit, resin and such by a lamination process, for example. Patterning is followed by baking at over 500xc2x0 C., in order to remove the resin contained in the paste or film and to fuse the silver particles and glass flit. Fusing of the fused silver particles raises their conductivity, and fusing of the glass flit affixes them to the front glass substrate.
After baking, the dielectric layer is formed. Powder from ground low-melting lead glass, resin, and solvent are mixed to form a past, which is applied by screen-printing or lamination to cover the silver electrodes. When the solvent has dried, the panel is baked at over 500xc2x0 C. a second time. At high temperature, the resin in the paste is removed and the low-melting lead glass is fused, forming the dielectric layer.
By the same processes, electrodes and a dielectric layer are formed and affixed to the back glass substrate as well.
In a PDP which uses silver electrodes, silver is ionized in the baking process and diffused inside the glass substrate, by reactions such as ion exchange with sodium included in the glass (usually 2.5 wt % to 15 wt %). This diffusion of silver is thought to proceed in proportion to the temperature and duration of baking. The diffused silver is reduced inside the glass substrate, forming colloid and causing yellowing of the glass. Yellowing in the front glass substrate is especially problematic, as it can cause deterioration of the color temperature and loss of image quality.
To reduce yellowing, suppression of silver ion diffusion by lowering baking temperature has been considered, but decomposition of the resin and softening of the electrode and dielectric layer materials are also dependant on the baking temperature, making change difficult. Similarly, reduction of baking time has been considered also. However, by simply shortening the baking time, resin may be left in the electrodes and dielectric layer, and fusion in these parts may be insufficient, carrying the risk of reduced electrode conductivity and reduced dielectric layer insulation.
This yellowing phenomenon occurs not only in PDPs, but also in field emission display panels and other display panels which have thick-film silver electrodes formed on a glass substrate, creating high demand for technology to suppress yellowing.
The objective of the current invention is to provide a display panel with reduced yellowing of the glass substrates, and a production method for such a display panel.
In order to achieve the objective stated above, the display panel of the current invention includes a first panel, which has rows of electrodes covered by a dielectric layer, and a second panel, which is arranged parallel to the first panel and separated from it by barrier ribs. The first panel is made of a material which includes glass, and its electrodes are made of a material which includes silver. The display panel is characterized by a ratio of the concentration of diffused silver in the dielectric layer, measured in an area of the dielectric layer with a diameter of 5 xcexcm centered 5 xcexcm from the interface of the electrode and the dielectric layer, versus the concentration measured in an area of the glass substrate with a diameter of 5 xcexcm centered 7.5 xcexcm from the interface of the electrode and the substrate, that is 0.5 or less.
With such a display panel, diffusion of silver into the dielectric layer is low, indicating that degradation of the dielectric layer is suppressed and diffusion of silver into the glass substrate is also low, resulting in reduced yellowing.
When the first panel is the front panel, deterioration of the display""s color temperature can be reduced also.
To reduce yellowing in the front panel, it is desirable for the concentration of silver in an area of the glass substrate with a diameter of 5 xcexcm centered 7.5 xcexcm from the interface of the electrode and the substrate to be 0.8 wt % or less.
It is also desirable for the concentration of silver in an area of the dielectric layer with a diameter of 5 xcexcm centered 5 xcexcm from the interface of the electrode and the substrate to be 0.4 wt % or less.
In order to achieve the objective stated above, the first panel is further characterized by a substrate containing glass and 2.5 wt % of sodium, as well as a ratio of the concentration of sodium in the glass substrate, measured in an area with a diameter of 5 xcexcm centered 7.5 xcexcm from the interface with the electrode, versus the concentration measured in an area of the opposite surface of the glass substrate with a diameter of 5 m, that is 90% or more.
Silver is thought to cause ion exchange with sodium contained in the glass, so that if a high concentration of sodium remains in the glass after baking, it is inferred that silver diffusion into the dielectric layer is low. Consequently, dielectric breakdown of the dielectric layer is suppressed, silver diffusion into the glass substrate is small, and yellowing of the glass substrate is reduced.
Here, if the concentration of sodium in the first panel, measured in an area of the glass substrate with a diameter of 5 xcexcm centered 3 xcexcm from the side of the electrode and 3 xcexcm from the interface of the electrode and the substrate, is kept to 0.25 wt % or less, then yellowing of the glass substrate will be limited.
Regarding the shape of the panels, it can be said that the substrate of the first panel is in direct contact with the rows of electrodes on it across the display area of the display panel.
The production method for the display panel according to the present invention is characterized by a panel forming step that involves creating silver electrodes on the substrate and forming a dielectric layer covering the electrodes. The panel forming step includes a first, a second and a third step. The first step involves forming a pattern layer on the substrate, using a first resin and a first glass material. The second step involves forming a coating layer, which covers the pattern layer formed in the first step, using a second resin and a second glass material. The third step involves simultaneous baking of the pattern layer and the coating layer. The third step involves a first, a second and a third step. In the first step, temperature is raised to begin decomposition of the first and second resin contained in the pattern layer and coating layer. In the second step, temperature is maintained above softening point of the first glass material but below softening point of the second glass material. In the third step, temperature is raised above softening point of the second glass material.
In the third step above, simultaneous baking of the pattern layer and the coating layer suppresses diffusion of silver into the substrate and the coating layer.
According to the above production method, gas emitted from the first dielectric glass during heating in the second step can pass through the second dielectric glass layer above, since the second layer is not yet softened. Therefore, bubbles are not trapped in the first dielectric glass layer, and providing a more dense silver electrode.
In the second step, it is also possible to maintain a temperature above the softening point of the first glass and below softening point of the second glass, by creating a period when the heat-up rate is slower than that of the first step.
Here in the third step, burn-off of the resin can be promoted by operating in low pressure, dry gas, or oxidizing gas environments, and silver oxidation can be suppressed by operating in a reductant gas environment.
The production method for the display panel according to the present invention is further characterized by a front panel forming step that involves creating silver electrodes on the front glass substrate and forming a dielectric layer covering the electrodes. The front panel forming step includes a first, a second and a third step. The first step involves forming a pattern layer on the front glass substrate, using silver, a first resin and a first glass material. The second step involves forming a coating layer, which covers the pattern layer formed in the first step, using a second resin and a second glass material. The third step involves simultaneous baking of the pattern layer and the coating layer.
By the method described above, simultaneous baking of the front panel allows for a reduction in baking time, resulting in reduced diffusion of silver into the substrate and allowing manufacture of a display panel with higher color temperature than by conventional methods.